Most types of consumer and commercial electronic equipment produced today require electrical power provided via alternating current (AC) power lines, which are typically referenced to earth ground. Such equipment typically employ power supplies that convert the voltages of the AC power lines to some level of direct current (DC) voltage to provide power of a more useful form to the internal circuits of the equipment. However, much of this equipment requires galvanic (i.e., DC) isolation between portions of their resident circuitry and earth ground for proper circuit operation and user safety. As a result, the power supplies employed by electronic equipment generally provide that isolation.
Such an isolated power supply ideally possesses a number of characteristics. The supply should accept a wide range of AC power line voltages while producing a stable, constant DC voltage. Hopefully, this function is performed with essentially no power loss, and without producing any undesirable side effects that adversely impact the performance of the associated equipment.
One such side effect commonly produced by power supplies is injected current, which is common-mode AC current induced in the DC outputs of a power supply due to coupling between the isolated and non-isolated portions of the supply. Typically, inter-winding capacitance in a power transformer within the power supply provides the major source of such coupling. As a result, the injected current produced depends to some degree on the magnitude of the inter-winding capacitance, as well as the magnitude, primary frequency, and harmonic content of the driving voltage applied at the terminals of the primary winding of the transformer.
For example, a traditional isolated power supply 100, as shown in FIG. 1, normally exhibits some level of injected current. An AC line voltage VACL, normally with an amplitude between 90 and 264 volts root-mean-square (RMS), drives the primary windings of a power isolation transformer 110, thereby inducing a smaller AC voltage VACT in the transformer secondary windings. This voltage then drives a rectifier 120, which converts the smaller AC voltage VACT into a half-wave or full-wave rectified voltage VACR. A low-pass filter 130 then converts the rectified voltage VACR into a DC voltage VDCR exhibiting some small AC “ripple.” In some cases, a voltage regulator 140 is then employed to generate a DC output voltage VDCO with reduced AC ripple, while also providing some DC voltage level stability against amplitude changes in the AC line voltage VACL.
Usually, the rather high amplitude of the AC line voltage VACL produces a high level of injected current by way of the aforementioned inter-winding capacitance of the power transformer 110. In order to mitigate the level of the injected current, special winding techniques and electrostatic shielding are employed to reduce the capacitance, but at the expense of a more costly transformer.
Additionally, the traditional power supply 100 normally exhibits low power efficiency. This characteristic is caused by a relatively low transformer primary-to-secondary winding ratio to ensure that the smaller AC voltage VACT is of a sufficiently high magnitude when the AC line voltage VACL is at a minimum. Therefore, when the AC line voltage VACL is higher than its minimum, the smaller AC voltage VACT is higher than necessary to provide the required DC output voltage VDCO, resulting in higher power dissipation, sometimes resulting in power efficiency as low as 50%, or less.
Furthermore, in order to produce the same DC voltage for a wide range of AC line voltages VACL, the transformer 110 often employs multiple windings and taps to allow different AC line voltages VACL to produce the same DC output voltage VDCO. The tap to be used in a particular circumstance is selected by way of a user-settable switch. While such a design lends flexibility with respect to the various AC power line voltages with which the associated equipment may be used, increased hardware costs and possible equipment damage due to an incorrect switch setting result.
To address this efficiency problem, a switch-mode DC-to-DC converter 200, as displayed in FIG. 2, may be employed as part of a larger power supply. First, an AC power line voltage (not shown) is converted to a relatively high DC input voltage VDCI by way of rectifiers and filter capacitors (also not shown), typically by way of a line voltage selector switch. The DC input voltage VDCI is then quickly switched ON and OFF intermittently via an electronic switch circuit 210 (typically comprising a set of transistors), resulting in a switched DC voltage VDCS. The switched voltage VDCS, alternating between the magnitude of the DC input voltage VDCI and zero volts (or the “open” state) is then applied to the primary winding of an isolation transformer 230 to generate a transformed DC switched signal VDCT at the secondary winding. This voltage is then rectified by a rectifier 240 to produce a rectified voltage VR, which may then be passed to a low-pass filter 250 to reduce any AC components of that voltage VR, thus resulting in a final DC output voltage VDCO.
The magnitude of the DC output voltage VDCO is influenced primarily by the operation of the electronic switch circuit 210, which is managed by a control circuit 220. More specifically, the higher the duty cycle of the switches (i.e., the longer they are in the closed or ON state), the higher the DC output voltage VDCO. Thus, the control circuit 220 monitors the DC output voltage VDCO to adjust the duty cycle properly, often through an isolation circuit 260 to maintain galvanic isolation between the AC input power line and the output of the DC-to-DC converter 200.
Many such converters 200 available today can tolerate a wide range of DC power line voltages without the need for multiple taps or a switch. Power supplies that utilize such a converter 200 are termed “universal-input” power supplies.
Another advantage of such a design is that the use of an electronic switch circuit 210 results in the transistors involved transitioning into either the ON (saturated) or OFF (non-conducting) state very quickly, resulting in very little power loss, making the DC-to-DC converter 200 quite efficient. However, this design also results substantially in a square wave for the switched DC voltage VDCS being applied directly to the transformer. Such a signal possesses a strong harmonic content at high frequencies, thus producing a significant amount of injected current into the non-isolated portion of the converter 200.
Also, to allow the use of a smaller transformer and other components, the control circuit 220 typically drives the switches at a much higher frequency (e.g., 20 kilohertz (kHz) to a few megahertz (MHz)) than that of an AC power line.
Unfortunately, such high frequencies easily couple though the inter-winding capacitance of the transformer 230, therefore augmenting the injected current produced. Additionally, the fact that the magnitude of the switched DC voltage VDCS is relatively high further exacerbates this phenomenon.
As a result, given the foregoing discussion, a need currently exists for an isolated DC-to-DC converter that exhibits improved reduction of injected current.